1. Field of the Invention
The present invention relates to electrostatic dissipative polymers, and in particular to salt-modified electrostatic dissipative polymers reacted from polyether oligomers. More particularly, the present invention is directed to salt-modified electrostatic dissipative polymers reacted from polyether oligomers, which exhibit relatively low surface and volume resistivities, but which are generally free of excessive amounts of extractable anions.
2. Background Art
The formation and retention of charges of static electricity on the surface of most plastics is well known. Plastic materials have a significant tendency to accumulate static electrical charges due to low electrical conductivity. This type of formation and retention of charges of static electricity can be problematic. The presence of static electrical charges on sheets of thermoplastic film, for example, can cause the sheets to adhere to one another thus making their separation for further processing more difficult. Moreover, the presence of static electrical charges causes dust to adhere to items packaged in a plastic bag for example, which may negate any sales appeal.
The increasing complexity and sensitivity of microelectronic devices makes the control of static discharge of particular concern to the electronics industry. Even a low voltage discharge can cause severe damage to sensitive devices. The need to control static charge buildup and dissipation often requires the entire assembly environment for these devices to be constructed of partially conductive materials. It also may require that electrostatic protective packages, tote boxes, casings, and covers be made from conductive polymeric materials to store, ship, protect, or support electrical devices and equipment.
The prevention of the buildup of static electrical charges which accumulate on plastics during manufacture or use has been accomplished by the use of various electrostatic dissipative (ESD) materials. These materials can be applied as a coating which may be sprayed or dip coated on the article after manufacture, although this method usually results in a temporary solution. Alternatively, these materials can be incorporated into a polymer used to make the article during processing, thereby providing a greater measure of permanence.
However, the incorporation of these lower molecular weight electrostatic dissipative materials (antistatic agents) into the various matrix or base polymers has its own limitations. For example, the high temperatures required for conventional processing of most polymers may damage or destroy the antistatic agents, thereby rendering them useless with respect to their ESD properties. Moreover, many of the higher molecular weight ESD agents are not miscible with the matrix or base polymers employed.
A large number of antistatic agents are also either cationic or anionic in nature. These agents tend to cause the degradation of plastics, particularly PVC, and result in discoloration or loss of physical properties. Other antistatic agents have significantly lower molecular weights than the base polymers themselves. Often these lower molecular weight antistatic agents possess undesirable lubricating properties and are difficult to incorporate into the base polymer. Incorporation of the lower molecular weight antistatic agents into the base polymers often will reduce the moldability of the base polymer because the antistatic agents can move to the surface of the plastic during processing and frequently deposit a coating on the surface of the molds, possibly destroying the surface finish on the articles of manufacture. In severe cases, the surface of the article of manufacture becomes quite oily and marbleized. Additional problems which can occur with lower molecular weight ESD agents are loss of their electrostatic dissipative capability due to evaporation, the development of undesirable odors, or promotion of stress cracking or crazing on the surface of an article in contact with the article of manufacture.
One of the known lower molecular weight antistatic agents is a homopolymer or copolymer oligomer of ethylene oxide. Generally, use of the lower molecular weight polymers of ethylene oxide or polyethers as antistatic agents are limited by the above-mentioned problems relative to lubricity, surface problems, or less effective ESD properties. Further, these low molecular weight polymers can be easily extracted or abraded from the base polymer thereby relinquishing any electrostatic dissipative properties, and in some instances can also produce undesirably large amounts of unwanted extractable anions, and in particular chlorine, nitrate, phosphate, and sulfate anions.
There are several examples of high molecular weight electrostatic dissipative agents in the prior art. In general, these additives have been high molecular weight polymers of ethylene oxide or a derivative thereof like propylene oxide, epichlorohydrin, glycidyl ethers, and the like. It has been a requirement that these additives be high molecular weight materials to overcome the problems mentioned above. However, these prior art ESD additives do not have a desired balance between electrical conductivity and acceptable low levels of extractable anions, in particular chlorine, nitrate, phosphate, and sulfate, which in turn can cause any manufactured articles containing such ESD additives to have unacceptable properties for some end uses.
The present invention solves the problem of obtaining electrostatic dissipative polymers or additives which exhibit relatively low surface and volume resistivities without unacceptably high levels of extractable anions, in particular chlorine, nitrate, phosphate, and sulfate anions. These electrostatic dissipative polymers in turn can be incorporated in base polymer compositions useful in the electronics industry without producing other undesirable properties in a finished article of manufacture.